In heavy fermion superconductors, much can be gained from understanding the magnetic mechanism that leads to electron pairing in certain materialsand how it can be so easily disrupted. As a result, researchers are now turning to a destructive approach to clue-finding: using impurity atoms to introduce waves of disorder in exotic electronic materials. Together with his colleagues, Samus Davisa Physicist at the US Department of Energy's (DOE's) Brookhaven National Laboratoryrecently utilized this destructive approach (see photo). The team combined it with an imaging tool and applied those approaches to a material that it has been studying. By substituting just a few atoms, the researchers found that they could disrupt the delicate interactions that give the material its unique properties.
The materiala compound of uranium, ruthenium, and siliconis known as a "heavy-fermion" system. Electrons traveling through the material periodically stop to interact with other electrons, which are localized on the uranium atoms making up the crystal framework. These stop-and-go magnetic interactions slow down the electrons, making them appear as if they've taken on extra mass.
In 2010, Davis and a group of collaborators visualized these heavy fermions using spectroscopic-imaging scanning tunneling microscopy (SI-STM), an approach measuring the wavelength of electrons of the material in relation to their energy. The idea of the present study is to "destroy" the heavy fermion system by substituting thorium for some of the uranium atoms.
Unlike uranium, thorium is a non-magnetic material. As a result, the electrons should be able to move freely around the thorium atoms. The areas where the electrons should flow freelyknown as "Kondo holes"do not support superconductivity. To help to explain why, Davis' team worked with thorium-doped samples.
The SI-STM measurements bore out an idea proposed by Physicist Dirk Morr of the University of Illinois. He predicted that electron waves would oscillate wildly around the Kondo holes. By destroying the heavy fermions, which must pair up in order for the material to act as a superconductor, the Kondo holes disrupt the material's superconductivity.